In medical diagnosis and in physiologic evaluation it is desirable to precisely quantitate respiratory flow over time. The usual methods employ a mouthpiece or a mask. These devices are not always tolerated by subjects (or patients) and even if tolerated, they interfere with respiration, the very parameter being measured. In addition, mouthpieces and masks limit freedom of movement, are inconvenient and uncomfortable, particularly when used over extended times.
These problems have been overcome by quantitating respiratory volume changes by measuring the movement of the rib cage and abdomen as a reflection of changes in lung volume. Two pairs of magnetometers have been placed on the surface of the rib cage and abdomen to quantitate anterior-posterior distance change. The magnetometer pairs produce a voltage proportional to their separation.
In addition a technique has been described that measures the movement of the rib cage and abdomen using insulated wire, held closely encircling the torso, whose inductance changes are proportional to the cross sectional area enclosed. This variable inductance is connected to a variable frequency LC oscillator which is in turn connected to a frequency to voltage convertor giving a voltage proportional to cross sectional area of the torso.
In both of these techniques the two voltages (from rib cage and abdomen) must be calibrated, i.e., they are properly weighted and summed to give a voltage proportional to instantaneous lung volume. The mathematical formulation of the calibration is in both instances based on the publication of Konno and Mead.
To measure rib cage or abdominal expansion, a moment actuated piezoelectric crystal pulled by an elastic belt worn around the chest or abdomen has been described in Reibold, U.S. Pat. No. 3,782,368. Other devices including strain gauges, mercury filled silastic rubber, pneumatic tubes, and tissue electrical impedance have been used. All of these devices are qualitative relative to actual dimension change. They either fail to be linear or fail to follow dimensional changes instantaneously.
The magnetometer technique requires bulky apparatus and undesirable limitation on the activity and movement of the subject. The inductance technique has a limited response time. It is further limited by the sensitivity of the LC oscillator to electric connection. The sensitivity of calibration is limited by movement of the inductance wire on the torso.
The Reibold moment piezoelectric crystal fails to provide means for a quantitative measurement. It is sensitive to position around the body and to distortion of the elastic belt. It requires significant limitation on the activity and movement of the subject, e.g., turning in bed. Finally, baseline drift requires predetermined and constant static belt tension.